Geology of Anglesey: A
journey through time
Part
1 Part 2 Part 3
Part 4
Part 4: Into modern times:
The final part of the journey....
The Old Red Sandstone
Continent
Part 3 ended with the collision of Avalonia with
Laurentia and Baltica to form yet another new
continent. Thus amalgamated, the large landmass
drifted on northwards throughout the Devonian
Period, with Anglesey finally crossing the
Equator during the latter part of the
Carboniferous Period. By Permian times, the rest
of Gondwana had moved north and collided with the
amalgamated Avalonia, Laurentia and Baltica; in
addition, Siberia had joined Baltica in the
collision that created the Ural Mountains. In
this way the supercontinent of Pangaea came
together. How did these events affect Anglesey?
During Devonian times, arid, terrestrial
conditions prevailed. The sediments which
accumulated vary from conglomerates consisting of
Precambrian quartzite pebbles in a red muddy
matrix, exposed at the base of Bodafon Mountain,
to red mudstones containing yellow evaporite
spots and patches (cornstones), typical of
dried-out lake deposits, as seen on the coast at
Lligwy Bay. Collectively they are referred to as
the Old Red Sandstone, and the rather hostile
environment they were deposited in was widely
developed across the UK, stuck firmly within what
has been termed the "Old Red Sandstone
Continent".
The sea returns
However, the terrestrial conditions of the
Devonian gave way to the sea once again in early
Carboniferous times, when, in warm, tropical
conditions and a shallow-sea environment, a thick
sequence dominated by limestones was deposited.
These beds were laid down in cycles as the sea
transgressed and retreated as many as 11 times
over North Wales. On Anglesey, only four of these
cycles, which consist of sandy beds followed by
limestone and then mudstone, were deposited on
this, the northern margin of the Wales-Brabant
Massif - the remains of Avalonia. The rocks are
rich in fossilised brachiopods and corals and
often exhibit palaeokarstic surfaces and rarer
sandstone pipes.
Amazonian Anglesey
Later Carboniferous strata are restricted in
their outcrop to a linear NE-SW-trending belt
which reaches the south-west coast at Malltraeth
Bay, and a second smaller area around Abermenai
Point. Westphalian coal-bearing strata were
formerly mined for coal beneath Malltraeth Marsh,
although they are of no economic interest today,
the seams being very narrow and the ground
waterlogged. Fossil remains of the giant
horsetail, Calamites
and tree-roots (Stigmaria),
as well as insect parts and fish scales have been
found in these deposits and their palaeoecology
demonstrates that, during the time of formation,
the environment was a tropical rain-forest
situated close to the Equator, in a similar
situation to the Amazon forests of today.
By Permian times, the formation of Pangaea was
complete. It straddled the globe, extending
towards both poles, with Anglesey lying just
north of the equator. Large continental
landmasses create climates with extreme
variations of heat and cold and highly seasonal
rainfall patterns, with desert conditions
widespread. The closing of the Palaeozoic era,
250 million years ago at the end of the Permian
Period, was characterised by inhospitable
conditions, further complicated by a probable
asterioud impact, which resulted in a tremendous
and wide-ranging mass-extinction with over 70% of
terrestrial and 90% of marine species
disappearing.
The missing strata......
Anglesey's journey northwards continued through
the following Mesozoic era (consisting of the
Triassic, Jurassic and Cretaceous Periods).
During this time, evidence from elsewhere in the
UK shows how conditions changed from the saline
desert lakes of the Triassic to the warm tropical
seas of the Jurassic and Cretaceous, which
covered low-lying areas. Triassic strata outcrop
on the sea-bed to the east of Anglesey, but
whether the sea ever covered the island again is
uncertain, although it is now suspected that the
Chalk sea of the Upper Cretaceous covered
virtually the entire UK. However, there are no
sedimentary rocks on Anglesey representing the
time from the end of the Carboniferous Period
through to the Miocene - a gap of over 260
million years.
The Atlantic opens
Breakup of Pangaea began about 170 million years
ago, in Jurassic times, and in the following 100
million years, Laurentia separated from the
amalgamated Baltica, Siberia and Africa (and the
rest of the microcontinents that make up Western
Europe), whilst the Gondwana part of Pangaea
split later in the Cretaceous Period and both
North and South America drifted westwards.
Incidentally, most of old Avalonia remained with
Western Europe, excepting the area of the modern
North American continent between New England and
Newfoundland, which consists of Avalonian rocks.
A new ocean - the Atlantic - was developing and
widening, with much volcanic activity, including
an intense episode of volcanism and associated
igneous intrusions, focussed along the western
seaboard of Scotland, early in the Cenozoic Era.
By this time, Anglesey lay between 40 –50o
north of the equator.
The contribution of the igneous activity, 55
million years ago, to the bedrock geology of
Anglesey was in the form of numerous basic dyke
intrusions. The dykes are rich in iron-bearing
silicate minerals, and are rather prone to deep
spheroidal weathering, so that their outcrops
often display the classic "onion-skin"
texture. Mapping of the dykes, by aeromagnetic
survey, has revealed a series of apparent offsets
that have been interpreted as evidence that
significant amounts of sinistral faulting have
occurred in North Wales in the last 55 million
years. However, because some of the dykes are
exposed at outcrop, these apparent offsets can be
examined in detail. The outcrops show that the
intrusions ‘skipped’ from following one
plane of weakness to another, and so the offsets
appear not to be due to later faulting. The
uncertainties require that the outcrops are
preserved and subjected to future investigation.
The Cenozoic Era also brought into existence a
mineral that is named after Anglesey. The lead
sulphate, anglesite, was first described by
Beudant in 1832, although the existence of the
mineral, previously known as "vitriol de
plomb", at Parys Mountain had been known for
many years. It formed during the deep, prolonged
subtropical weathering that the climate permitted
at the time, during which the sulphide orebodies
were deeply oxidised into a thick
"gossan", much of which survived later
erosional processes and was instead removed by
the miners in their search for copper ore.
The recent discovery of Miocene deposits in
northern Anglesey could indicate that a
substantial Miocene cover once existed, but was
largely eroded following Plio-Pleistocene uplift.
It seems likely that the Menaian surface (and the
adjacent Snowdonian massif) were well-established
landscape features before the end of the Miocene.
The few areas of upstanding relief may represent
inselbergs exhumed from a deep saprolitic cover.
Sudden catastrophic Climate
Change
By late Cenozoic times, the UK was pretty much
where it is now in relation to the Poles and the
Equator. But there was one last sting in the
tail. A mere 1.6 million years ago, Earth went
from its stable warm climate into a series of
violent climatic oscillations - the ice-ages and
intervening warm periods of the Quaternary.
Anglesey contains an exceptional range of glacial
geomorphological features and sedimentary
deposits. Although the gross morphology of
Anglesey’s land surface is pre-glacial in
origin, its landscape was modified considerably
during the Pleistocene glaciations and the island
is now dominated by an erosion surface cut across
the older rocks.
The Quaternary has featured at least four
intensely cold periods during which large parts
of upland Britain were covered by ice sheets for
long periods. Although Anglesey was probably
overrun by ice on these occasions, only evidence
from the last major glacial phase - the Late
Devensian - is known. During that glaciation,
around 20,000 years ago, Anglesey was completely
submerged by ice. Two ice sheets from different
sources were involved. The Snowdonian mountains
were the source of ice streams that moved broadly
northwards towards Anglesey, while the massive
Irish Sea ice sheet, fed by glaciers from
Scotland, Ireland and Cumbria, moved onto the
island from the north. The Irish Sea ice stream
was dominant, and travelled north-east to
south-west across the island, broadly in keeping
with its NE-SW-trending, structurally controlled
rock ridges. The Welsh and Irish Sea ice streams
met in the region of the present-day Menai Strait
and produced a confluent south-westward flow.
Deposits from the Irish Sea ice tend to contain a
wide range of rock types from its diverse source
areas and from the varied geology of the seafloor
traversed. A red colouration is common, being
derived partly from Permian and Triassic rocks
offshore. The Irish Sea sediments commonly
contain unconsolidated seafloor debris, including
sand and shell fragments, dredged from the
seafloor by the ice. Tertiary lignite, coal
fragments and flint are also a characteristic
component of the Irish Sea deposits.
Alternatively, deposits from the Welsh ice sheet
reflect the geology of its source areas, with a
high proportion of Cambrian slates and mudstones,
varied Ordovician igneous materials and a
blue-grey colouration.
And so to the present.....
Landscape development during the last 10,000
years (post-glacial or Holocene) saw the
re-establishment of vegetation cover, the
development of soils and progressive sea-level
rise. Although the broad form of Anglesey’s
coastline can be traced back to Permo-Triassic
times, the soft-sediment features seen today are
largely an inheritance of the Quaternary,
particularly the present marine cycle which has
cliffed glacial deposits and reworked them into
estuarine mudflats, saltmarshes and sand dunes.
Recent borehole studies in the Menai Strait have
revealed one of the most comprehensive records of
sea-level change yet known in Europe. Extensive
basin mires with deep stratigraphic sequences
complement the marine evidence.
Now follow the link to the Geosites section of
the site, to find out where you can see the
evidence for this fascinating and complex story!
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Above: on the eastern coast of Anglesey, Old Red
Sandstone crops out near to the popular beach at
Traeth Lligwy. The characteristic red-brown
colour is evident. Photo: Stewart Campbell.
Above: that part of the geological time-scale
from the end of the Precambrian, 540 million
years ago, up to the present day, consisting of
one eon - the Phanerozoic, which is subdivided
into three eras, such as "Mesozoic" and
then into periods, such as "Jurassic".
Graphic: John Mason.
Above: thinly bedded greyish limestones of Lower
Carboniferous age, cropping out on the coast near
Penmon. Photo: Stewart Campbell.
Above: that part of the geological time-scale
from the end of the Mesozoic (the "K-T
Boundary"), 65 million years ago, up to the
present day, consisting of one era - the Cenozoic
- which is subdivided into two periods, and then
into epochs, such as "Eocene". The
current epoch, the Holocene, began only 0.01178
million years ago, so that it is too small to
show up on the chart. The Quaternary consists of
the Pleistocene and Holocene lumped together.
Graphic: John Mason.
Above: an early Cenozoic dyke cutting older rocks
on the coast at Porth Namarch, near Holyhead.
Basic in composition, with a lot of iron-bearing
silicates, the dyke-rock shows the distinctive,
spheroidal "onion-skin" weathering
pattern. Photo: Stewart Campbell.
Above: till - deposited by the Irish Sea
ice-sheet, about 20,000 years ago - is now being
reworked by the sea at Beaumaris - part of the
seemingly endless cycle of sedimentary erosion
and deposition. Photo: Stewart Campbell.
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